288 SUPERFICIAL HEAT. [SECT. xxvi. 



the direct heating influence of the solar rays is greatest at 

 the equator, and that it diminishes gradually as the latitude 

 increases. At the equator the maximum is 48|, while in 

 Europe it has never exceeded 29^. 



M. Pouillet has estimated with singular ingenuity, from a 

 series of observations made by himself, that the whole quan- 

 tity of heat which the earth receives annually from the sun 

 is such as would be sufficient to melt a stratum of ice 

 covering the whole globe 46 feet deep. Part of this heat is 

 radiated back into space ; but by far the greater part descends 

 into the earth during the summer, towards the zone of uni- 

 form temperature, whence it returns to the surface in the 

 course of the winter, and tempers the cold of the ground and 

 the atmosphere in its passage to the ethereal regions, where 

 it is lost, or rather where it combines with the radiation from 

 the other bodies of the universe in maintaining the tempera- 

 ture of space. The sun's power being greatest between the 

 tropics, the caloric sinks deeper there than elsewhere, and 

 the depth gradually diminishes towards the poles ; but the 

 heat is also transmitted laterally from the warmer to the 

 colder strata north and south of the equator, and aids in 

 tempering the severity of the polar regions. 



The mean heat of the earth, above the stratum of constant 

 temperature, is determined from that of springs ; and, if the 

 spring be on elevated ground, the temperature is reduced by 

 computation to what it would be at the level of the sea, as- 

 suming that the heat of the soil varies according to the same 

 law as the heat of the atmosphere, which is about 1 of Fah- 

 renheit's thermometer for every 3337 feet. From a com- 

 parison of the temperature of numerous springs with that 

 of the air, Sir David Brewster concludes that there is a par- 

 ticular line passing nearly through Berlin, at which the 

 temperature of springs and that of the atmosphere coincide ; 

 that in approaching the arctic circle the temperature of 

 springs is always higher than that of the air, while, pro- 

 ceeding towards the equator, it is lower. 



